PSMA has received considerable attention since its discovery (Horoszewicz et al. (1987) Anticancer Res. 7:927-35), mostly due to its specifically elevated expression by malignant prostate cells (Israeli et al. (1994) Cancer Res. 54:1807-11; Wright et al. (1996) Urology 48:326-34). Moreover, normal prostate cells predominantly express the cytosolic form while malignant cells express the full-length membrane bound form (Su et al. (1995) Cancer Res. 55:1441-3). PSMA expression was also reported, albeit at lower levels, in the brain, kidney, salivary gland and duodenum. (e.g. Renneberg et al. (1999) Urol. Res. 27(1):23-7; Troyer et al. (1995) Int. J. Cancer 62(5):552-8; Israel et al. (1994) Cancer Res. 54(7):1807-11; Israel et al. (1993) Cancer Res. 53(2):227-30). These and all other extrinsic materials discussed herein are incorporated by reference in their entirety. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply
PSMA is a 750-amino acid type II transmembrane protein with carboxypeptidase activity (specificity: N-acetyl-asp-glu). Remarkably, the majority of the protein is present in the extracellular space and so forms an attractive target for diagnostic and therapeutic agents. For example, monoclonal antibodies can be raised against PSMA and modified for use in diagnostic imaging (Sodee et al. (1998) Prostate 37:140-8; Lopes et al. (1990) Cancer Res. 50:6423-6429; Elgamal et al. (1998) Prostate 37(4):261-9; Lamb and Faulds (1998) Drugs Aging 12(4):293-304). Since PSMA is also expressed in neovascular endothelial cells in a variety of cancers (Chang et al. (1999) Clin. Cancer Res. 5:2674-81; Liu et al. (1997) Cancer Res. 57:3629-34), PSMA antibodies can also be used in tumor vascular imaging and anti-angiogenesis therapy. However, in vivo use of monoclonal antibodies is often limited to due various factors. Most significantly, antibodies are relatively large molecules and often problematic in terms of large-scale production and isolation. Moreover, and especially where antibodies are used over an extended period, many antibodies will ultimately elicit an immune response. Still further, even though most antibodies have fairly strong affinity towards their antigen, the high molecular weights of antibodies requires relatively large dosages for tagging or otherwise labeling cells with antibodies.
To circumvent problems associated with antibodies, antibody fragments or single chain variable chain fragments (scFvs) can be employed. While such approaches advantageously reduce the molecular weight and in at least some instances overcome limited production volume, numerous problems nevertheless remain. Among other things, purification of scFvs is typically performed from a recombinant source, which introduces a new set of potentially antigenic components. Alternatively, where antibody fragments are used, (e.g., Fab or F(ab′)2), protease activity is often carried over to the preparation of antibody fragments, which is highly undesirable.
Alternatively, non-antibody ligands can be used as described in U.S. Pat. No. 6,933,114 where high-affinity RNA aptamers were produced in a SELEX (Systematic Evolution of Ligands by EXponential Enrichment) process to so generate binders to PSMA. Remarkably, the inventors reported various high-affinity nuclease resistant RNA sequences that inhibited (at low nM Ki) the peptidase activity of PSMA. While such molecules overcome at least some of the difficulties associated with antibodies, other problems tend to arise. For example, as such RNA aptamers are often relatively long RNA molecules (e.g., 65-70 bases, and even longer), conformational changes may reduce utility of such molecules. Further, and at least in most in vivo conditions, large RNA molecules are typically bound or otherwise associated with various serum proteins and so rendered ineffective for binding to PSMA, particularly where nucleic acid analogs are used (e.g., phosphorothioate) to stabilize the molecule against degradation.
Thus, while numerous PSMA ligands are known in the art, there is still a need to provide improved PSMA ligand motifs and/or ligands, compositions, and methods using such improved PSMA ligand motifs and/or ligands.